Mass spectrometers operate by selectively deflecting the trajectories of charged particles using electric and magnetic fields. An electric charge is induced on the constituent molecules of the sample to be analyzed in an ion source. There are many forms of ion source ionization such as electrospray, thermospray, corona discharge, fast atom bombardment, particle beam, chemical and electron impact. Chemical ionization, particle beam and electron impact ionization rely on the production of an electron beam which is accelerated to a chosen translational kinetic energy and directed to pass into and through a region of the ion source which contains the constituent molecules of the gaseous sample to be analyzed.
The electron beam is usually generated by passing an electric current through a refractory metal wire. The current heats the wire to a temperature where thermionic emission of electrons occurs. The filament is typically held in an electric field so that emitted electrons are accelerated from the hot filament in the direction of the gradient of the electric field. An axial magnetic field is used to constrain the motion of the electrons to a narrow beam. Any component of electron motion which is perpendicular to the magnetic lines of force acts to deflect the electrons into a spiral trajectory. The acceleration of the electrons through the electric field sets the translational kinetic energy of the electrons in the electron beam as they spiral along the magnetic lines of force through the ionization region. The translational energy affects the nature of the interaction between the gaseous sample molecules or particles and the electrons.
In an ion source for a magnetic sector or quadrupole mass spectrometer or an external ion source for ion injection into an ion trap, for example, the ions produced by interaction between the electron beam and the neutral sample molecules are extracted from the ionization region by a potential gradient between the ionization region and the mass analyzer.
Although the basics of ion source design draw upon fundamental principles, an ion source's quantitative performance depends upon the interaction of many subtle design characteristics. Ion source stability and sensitivity are important measures of a design, as are filament lifetime, and ion source serviceability. The electrons emitted from a filament are only useful if they pass through the ionization region and this has a significant effect on ion source sensitivity. A known design used for many years includes a hairpin-shaped filament. By using a hairpin-shaped filament and enclosing all but the tip of the hairpin in a metal shroud, electron emission can be limited to the exposed tip. Electrons which are emitted by the filament wire within the shroud pass to the shroud, which is electrically connected to one of the filament leads.
There are several problems associated with the filament assemblies used in electron impact or chemical ionization source. First, the filament produces a large amount of heat which may cause the temperature of the ionization source to reach undesirable temperatures or may cause uneven heating of the ionization region. Second, the high temperature of the filament causes the refractory metal of the filament to sublime. This creates weak spots in the filament and premature failure. Third, the high electron density or space charge of the electron beam creates a negative potential well. This potential well creates an alternate path for positive ions formed in the ionization region. The positive ions follow the electron beam back to the filament; this reduces the sensitivity of the ion source since those ions never enter the mass analyzer. Fourth, the ions which follow the electron beam back to the filament may bombard the filament, thus sputtering metal off the filament and shortening filament lifetime. Fifth, ions may deposit carbon residue in the filament region which may build up to form conductive whiskers that electrically short the filament to the ion source. Sixth, the filament wire is fragile. Typically, the filament assembly is removable. This wire is easily bent broken when the filament assembly is removed. Seventh, a change in the desired electron energy via a change in the filament bias changes the electric gradient at the filament. In the presence of a fixed magnetic field, this changes the electron trajectories and the overall efficiency of electron delivery to the ionization region.